The steadystate superconducting tokamak (SST1) is aimed to demonstrate long pulse plasma discharges employing non-inductive current drive by means of lower hybrid current drive (LHCD) system. The major and minor radius of the machine is 1.1m and 0.2m respectively. The LHCD system for SST1 comprises of klystrons, each rated for 0.5MW-CW rf power at a frequency of 3.7 GHz. The grill antenna comprises of two rows, each row accommodating 32 waveguide elements. Electron cyclotron resonance (ECR) breakdown assisted Ohmic plasma is formed in SST1 to overcome the issues associated with low loop voltage start-ups. With recent modifications in the poloidal coils configuration, even with narrow EC pulse (~50ms), good repeatable and consistent Ohmic plasmas could be produced which helped in carrying out LHCD current drive experiments on SST1. These experiments demonstrated both fully as well as partially driven non-inductive plasma current in SST1 tokamak. Discharges with zero loop voltages were obtained. The interaction of lower hybrid waves with plasma and generation of suprathermal electrons could be established using energy spectra measured by CdTe detectors. Various other signatures like drop in loop voltages, negative loop voltages, spikes in hard x-rays and increase in 2nd harmonic ECE signal, further confirmed the current drive by LHW’s. The beneficial effect of LHW’s in suppressing hard x-rays was also demonstrated in these experiments. The non-inductive current drive in SST1 could also be established by modulating LH power. The longest discharge of ~650ms could be obtained in SST1 with the help of LHW’s. In this paper, the experimental results obtained with LHCD experiments on SST1 is reported and discussed in more details.
The successful operation of superconducting tokamaks is very challenging because of limitations arising out of a complex geometry, configuration and construction. The slow penetration of loop voltage because a continuous plasma chamber requires low loop voltage plasma initiation. Apart from these operational challenges, SST1 also has technical issues related to cooling down of the superconducting poloidal field (PF) coils, for obtaining advanced plasma operation and control. Furthermore, limits are imposed on the loop voltage by magnet power supplies and PF coil insulation. To improve our understanding of these issues, operations in SST-1 have been attempted with low loop voltages and will be discussed in this paper.It is envisaged that designing and carrying out experiments in SST1, to understand above issues, may be very complex, time consuming and may not be feasible due to complexities in SST1 machine operational scenario. Hence Aditya machine upgradation was initiated with the understanding that solution to some of these problems may be obtained through experiments specially designed for this purpose in ADITYA-U, in parallel. Thus some of these experiments are also being planned to carry out in tokamak ADITYA-U, in particular, non-inductive current drive using lower hybrid waves, employing passive active multi-junction (PAM) antenna. The performance of the lower hybrid waves launched by the grill antenna is presented and the need for PAM antenna is presented. The design of the PAM antenna for ADITYA-U machine is also discussed.
To drive plasma current non inductively, a Lower Hybrid Current Drive (LHCD) system with a passive-active multijunction (PAM) antenna for injecting lower hybrid wave (LHWs) has been designed, fabricated and to be integrated with ADITYA-U Tokamak. A fast electron population in the energy range of a few keV to several hundreds of keV is generated with the injection of the LHWs. These fast electrons interacting mainly with ions and electrons result in Hard X-Ray (HXR) emission called Fast Electron Bremsstrahlung acronym as FEB emission. A single channel FEB detection system is being used in the earlier experiment of LHCD in ADITYA-U tokamak. However, the single channel detection system can't provide a radial emissivity profile of HXR intensity distribution. In order to determine a radial emissivity profile, for the first time, a multichannel FEB detection system is designed for ADITYA-U tokamak. This paper describes the detailed conceptual design for the selection of suitable detectors, optimization of the collimators, shielding geometry, and low energy filtering cut-off. In the present design analysis, the soller collimator concept is considered over a simple pinhole camera system due to its several advantages. In order to optimize the multichannel FEB system parameters for the best possible performance, a forwarded modelling code has been used. The signal to background ratio at each detector location has been estimated for a few system parameters and reported herein.
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